The present invention relates to an optical fiber coil unit with an optical fiber wound on a bobbin for use in, for example, an optical fiber sensor of a fiber optic gyro.
FIG. 1 shows a conventional optical fiber coil unit for use in a fiber optic gyro. As shown, the conventional optical fiber unit has an optical fiber coil 12 wound on a cylindrical bobbin 11 mounted on a flat support plate 13. When a rotational angular rate is applied to the optical fiber coil 12 through which clockwise and counterclockwise light beams are propagating, a phase difference arises between the two light beams. By electrically detecting the phase difference, the applied angular rate is measured.
It is well-known in the art that when a sensing portion (an optical fiber coil) of a gyro using a ring-shaped optical fiber coil undergoes a nonuniform temperature change, a phase difference is caused between the clockwise and counterclockwise light beams propagating through the optical fiber of the sensing portion, developing a drift in the gyro output.
As a solution to this problem, there is proposed in Japanese Patent Application Laid-Open Gazette No. 80581/89 a construction wherein both end portions of an optical fiber wound around a bobbin are disposed adjacent to each other so that they are equally influenced by heat.
With such a structure, however, since respective layers of the coil are wound alternately, the winding end portion of the optical fiber layer crosses over the underlying optical fiber layer and a stress is induced in the optical fiber at the crossover portion, and consequently, a substantial ambient temperature change will cause a marked change in the light propagation characteristic of the stress-induced portion, causing a substantial drift in the gyro output.
Moreover, even if the optical fiber is wound in a symmetrical configuration with its opposite ends disposed adjacent to each other, a time lag occurs in heat conduction due to the heat conductivity of the optical fiber itself until symmetrical portions of the fiber reach the same temperature, and in this lag time, a difference is caused between the propagation characteristics for the clockwise and counterclockwise light beams, resulting in a drift of the gyro output.
Thus, even such various optical fiber winding structures cannot satisfactorily reduce the drift in the gyro output which is caused by a temperature change. From the viewpoint of making the temperature change in the optical fiber uniform throughout it, it has been customary to make the coil bobbin 11 of a metal of high thermal conductivity, such as aluminum (Al), so that the optical fiber coil 12 is subjected to as uniform temperature as possible over the entire length thereof, but this attempt has also failed to reduce the drift in the gyro output due to an ambient temperature change.